@item C++ Language Options
@xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
@gccoptlist{-fabi-version=@var{n} -fno-access-control @gol
--fargs-in-order=@var{n} -fcheck-new @gol
--fconstexpr-depth=@var{n} -ffriend-injection @gol
+-faligned-new=@var{n} -fargs-in-order=@var{n} -fcheck-new @gol
+-fconstexpr-depth=@var{n} -fconstexpr-loop-limit=@var{n} @gol
+-ffriend-injection @gol
-fno-elide-constructors @gol
-fno-enforce-eh-specs @gol
-ffor-scope -fno-for-scope -fno-gnu-keywords @gol
-Wabi=@var{n} -Wabi-tag -Wconversion-null -Wctor-dtor-privacy @gol
-Wdelete-non-virtual-dtor -Wliteral-suffix -Wmultiple-inheritance @gol
-Wnamespaces -Wnarrowing @gol
--Wnoexcept -Wnon-virtual-dtor -Wreorder @gol
+-Wnoexcept -Wnon-virtual-dtor -Wreorder -Wregister @gol
-Weffc++ -Wstrict-null-sentinel -Wtemplates @gol
-Wno-non-template-friend -Wold-style-cast @gol
-Woverloaded-virtual -Wno-pmf-conversions @gol
-fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]} @gol
-fdiagnostics-color=@r{[}auto@r{|}never@r{|}always@r{]} @gol
-fno-diagnostics-show-option -fno-diagnostics-show-caret @gol
--fdiagnostics-parseable-fixits}
+-fdiagnostics-parseable-fixits -fdiagnostics-generate-patch}
@item Warning Options
@xref{Warning Options,,Options to Request or Suppress Warnings}.
-pedantic-errors @gol
-w -Wextra -Wall -Waddress -Waggregate-return @gol
-Wno-aggressive-loop-optimizations -Warray-bounds -Warray-bounds=@var{n} @gol
--Wno-attributes -Wbool-compare -Wno-builtin-macro-redefined @gol
--Wc90-c99-compat -Wc99-c11-compat @gol
+-Wno-attributes -Wbool-compare -Wbool-operation @gol
+-Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat @gol
-Wc++-compat -Wc++11-compat -Wc++14-compat -Wcast-align -Wcast-qual @gol
-Wchar-subscripts -Wclobbered -Wcomment -Wconditionally-supported @gol
-Wconversion -Wcoverage-mismatch -Wno-cpp -Wdangling-else -Wdate-time @gol
-Wno-div-by-zero -Wdouble-promotion -Wduplicated-cond @gol
-Wempty-body -Wenum-compare -Wno-endif-labels @gol
-Werror -Werror=* -Wfatal-errors -Wfloat-equal -Wformat -Wformat=2 @gol
--Wno-format-contains-nul -Wno-format-extra-args -Wformat-nonliteral @gol
+-Wno-format-contains-nul -Wno-format-extra-args -Wformat-length=@var{n} @gol
+-Wformat-nonliteral @gol
-Wformat-security -Wformat-signedness -Wformat-y2k -Wframe-address @gol
-Wframe-larger-than=@var{len} -Wno-free-nonheap-object -Wjump-misses-init @gol
-Wignored-qualifiers -Wignored-attributes -Wincompatible-pointer-types @gol
--Wimplicit -Wimplicit-function-declaration -Wimplicit-int @gol
--Winit-self -Winline -Wno-int-conversion @gol
+-Wimplicit -Wimplicit-fallthrough -Wimplicit-function-declaration @gol
+-Wimplicit-int @gol
+-Winit-self -Winline -Wno-int-conversion -Wint-in-bool-context @gol
-Wno-int-to-pointer-cast -Winvalid-memory-model -Wno-invalid-offsetof @gol
-Winvalid-pch -Wlarger-than=@var{len} @gol
-Wlogical-op -Wlogical-not-parentheses -Wlong-long @gol
-fgcse-sm -fhoist-adjacent-loads -fif-conversion @gol
-fif-conversion2 -findirect-inlining @gol
-finline-functions -finline-functions-called-once -finline-limit=@var{n} @gol
--finline-small-functions -fipa-cp -fipa-cp-clone -fipa-cp-alignment @gol
+-finline-small-functions -fipa-cp -fipa-cp-clone -fipa-cp-alignment -fipa-bit-cp @gol
-fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-icf @gol
-fira-algorithm=@var{algorithm} @gol
-fira-region=@var{region} -fira-hoist-pressure @gol
-fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss @gol
-fomit-frame-pointer -foptimize-sibling-calls @gol
-fpartial-inlining -fpeel-loops -fpredictive-commoning @gol
--fprefetch-loop-arrays @gol
+-fprefetch-loop-arrays -fprintf-return-value @gol
-fprofile-correction @gol
-fprofile-use -fprofile-use=@var{path} -fprofile-values @gol
-fprofile-reorder-functions @gol
-fstrict-overflow -fthread-jumps -ftracer -ftree-bit-ccp @gol
-ftree-builtin-call-dce -ftree-ccp -ftree-ch @gol
-ftree-coalesce-vars -ftree-copy-prop -ftree-dce -ftree-dominator-opts @gol
--ftree-dse -ftree-forwprop -ftree-fre -ftree-loop-if-convert @gol
+-ftree-dse -ftree-forwprop -ftree-fre -fcode-hoisting -ftree-loop-if-convert @gol
-ftree-loop-if-convert-stores -ftree-loop-im @gol
-ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns @gol
-ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize @gol
-ftree-switch-conversion -ftree-tail-merge -ftree-ter @gol
-ftree-vectorize -ftree-vrp -funconstrained-commons @gol
-funit-at-a-time -funroll-all-loops -funroll-loops @gol
--funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops @gol
+-funsafe-math-optimizations -funswitch-loops @gol
-fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model -fvpt @gol
-fweb -fwhole-program -fwpa -fuse-linker-plugin @gol
--param @var{name}=@var{value}
-fprofile-report @gol
-frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
-fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose @gol
--fstats -fstack-usage -ftime-report @gol
+-fstats -fstack-usage -ftime-report -ftime-report-details @gol
-fvar-tracking-assignments-toggle -gtoggle @gol
-print-file-name=@var{library} -print-libgcc-file-name @gol
-print-multi-directory -print-multi-lib -print-multi-os-directory @gol
-mneon-for-64bits @gol
-mslow-flash-data @gol
-masm-syntax-unified @gol
--mrestrict-it}
+-mrestrict-it @gol
+-mpure-code}
@emph{AVR Options}
@gccoptlist{-mmcu=@var{mcu} -maccumulate-args -mbranch-cost=@var{cost} @gol
-mcompat-align-parm -mno-compat-align-parm @gol
-mupper-regs-df -mno-upper-regs-df -mupper-regs-sf -mno-upper-regs-sf @gol
-mupper-regs-di -mno-upper-regs-di @gol
--mupper-regs -mno-upper-regs -mmodulo -mno-modulo @gol
+-mupper-regs -mno-upper-regs @gol
-mfloat128 -mno-float128 -mfloat128-hardware -mno-float128-hardware @gol
--mpower9-fusion -mno-mpower9-fusion -mpower9-vector -mno-power9-vector @gol
--mpower9-dform -mno-power9-dform -mlra -mno-lra}
+-mgnu-attribute -mno-gnu-attribute @gol
+-mlra -mno-lra}
@emph{RX Options}
@gccoptlist{-m64bit-doubles -m32bit-doubles -fpu -nofpu@gol
Turn off all access checking. This switch is mainly useful for working
around bugs in the access control code.
-@item -fargs-in-order
-@opindex fargs-in-order
-Evaluate function arguments and operands of some binary expressions in
-left-to-right order, and evaluate the right side of an assignment
-before the left side, as proposed in P0145R2. Enabled by default with
-@option{-std=c++1z}. @option{-fargs-in-order=1} implements all of the
-ordering requirements except function arguments.
+@item -faligned-new
+@opindex faligned-new
+Enable support for C++17 @code{new} of types that require more
+alignment than @code{void* ::operator new(std::size_t)} provides. A
+numeric argument such as @code{-faligned-new=32} can be used to
+specify how much alignment (in bytes) is provided by that function,
+but few users will need to override the default of
+@code{alignof(std::max_align_t)}.
@item -fcheck-new
@opindex fcheck-new
constant expression evaluation. The minimum specified by the standard
is 512.
+@item -fconstexpr-loop-limit=@var{n}
+@opindex fconstexpr-loop-limit
+Set the maximum number of iterations for a loop in C++14 constexpr functions
+to @var{n}. A limit is needed to detect infinite loops during
+constant expression evaluation. The default is 262144 (1<<18).
+
@item -fdeduce-init-list
@opindex fdeduce-init-list
Enable deduction of a template type parameter as
The C++ standard allows an implementation to omit creating a temporary
that is only used to initialize another object of the same type.
Specifying this option disables that optimization, and forces G++ to
-call the copy constructor in all cases.
+call the copy constructor in all cases. This option also causes G++
+to call trivial member functions which otherwise would be expanded inline.
+
+In C++17, the compiler is required to omit these temporaries, but this
+option still affects trivial member functions.
@item -fno-enforce-eh-specs
@opindex fno-enforce-eh-specs
enumerators). This assumption may not be valid if the program uses a
cast to convert an arbitrary integer value to the enumerated type.
+@item -fstrong-eval-order
+@opindex fstrong-eval-order
+Evaluate member access, array subscripting, and shift expressions in
+left-to-right order, and evaluate assignment in right-to-left order,
+as adopted for C++17. Enabled by default with @option{-std=c++1z}.
+@option{-fstrong-eval-order=some} enables just the ordering of member
+access and shift expressions, and is the default without
+@option{-std=c++1z}.
+
@item -ftemplate-backtrace-limit=@var{n}
@opindex ftemplate-backtrace-limit
Set the maximum number of template instantiation notes for a single
class through a pointer to the class itself or base class. This
warning is automatically enabled if @option{-Weffc++} is specified.
+@item -Wregister @r{(C++ and Objective-C++ only)}
+@opindex Wregister
+@opindex Wno-register
+Warn on uses of the @code{register} storage class specifier, except
+when it is part of the GNU @ref{Explicit Register Variables} extension.
+The use of the @code{register} keyword as storage class specifier has
+been deprecated in C++11 and removed in C++17.
+Enabled by default with @option{-std=c++1z}.
+
@item -Wreorder @r{(C++ and Objective-C++ only)}
@opindex Wreorder
@opindex Wno-reorder
The default @env{GCC_COLORS} is
@smallexample
-error=01;31:warning=01;35:note=01;36:caret=01;32:locus=01:quote=01
+error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:quote=01:\
+fixit-insert=32:fixit-delete=31:\
+diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32
@end smallexample
@noindent
where @samp{01;31} is bold red, @samp{01;35} is bold magenta,
-@samp{01;36} is bold cyan, @samp{01;32} is bold green and
-@samp{01} is bold. Setting @env{GCC_COLORS} to the empty
-string disables colors.
+@samp{01;36} is bold cyan, @samp{32} is green, @samp{34} is blue,
+@samp{01} is bold, and @samp{31} is red.
+Setting @env{GCC_COLORS} to the empty string disables colors.
Supported capabilities are as follows.
@table @code
@vindex note GCC_COLORS @r{capability}
SGR substring for note: markers.
-@item caret=
-@vindex caret GCC_COLORS @r{capability}
-SGR substring for caret line.
+@item range1=
+@vindex range1 GCC_COLORS @r{capability}
+SGR substring for first additional range.
+
+@item range2=
+@vindex range2 GCC_COLORS @r{capability}
+SGR substring for second additional range.
@item locus=
@vindex locus GCC_COLORS @r{capability}
@item quote=
@vindex quote GCC_COLORS @r{capability}
SGR substring for information printed within quotes.
+
+@item fixit-insert=
+@vindex fixit-insert GCC_COLORS @r{capability}
+SGR substring for fix-it hints suggesting text to
+be inserted or replaced.
+
+@item fixit-delete=
+@vindex fixit-delete GCC_COLORS @r{capability}
+SGR substring for fix-it hints suggesting text to
+be deleted.
+
+@item diff-filename=
+@vindex diff-filename GCC_COLORS @r{capability}
+SGR substring for filename headers within generated patches.
+
+@item diff-hunk=
+@vindex diff-hunk GCC_COLORS @r{capability}
+SGR substring for the starts of hunks within generated patches.
+
+@item diff-delete=
+@vindex diff-delete GCC_COLORS @r{capability}
+SGR substring for deleted lines within generated patches.
+
+@item diff-insert=
+@vindex diff-insert GCC_COLORS @r{capability}
+SGR substring for inserted lines within generated patches.
@end table
@item -fno-diagnostics-show-option
An empty range (e.g. ``45:3-45:3'') indicates that the string is to
be inserted at the given position.
+@item -fdiagnostics-generate-patch
+@opindex fdiagnostics-generate-patch
+Print fix-it hints to stderr in unified diff format, after any diagnostics
+are printed. For example:
+
+@smallexample
+--- test.c
++++ test.c
+@@ -42,5 +42,5 @@
+
+ void show_cb(GtkDialog *dlg)
+ @{
+- gtk_widget_showall(dlg);
++ gtk_widget_show_all(dlg);
+ @}
+
+@end smallexample
+
+The diff may or may not be colorized, following the same rules
+as for diagnostics (see @option{-fdiagnostics-color}).
+
@end table
@node Warning Options
@gccoptlist{-Waddress @gol
-Warray-bounds=1 @r{(only with} @option{-O2}@r{)} @gol
-Wbool-compare @gol
+-Wbool-operation @gol
-Wc++11-compat -Wc++14-compat@gol
-Wchar-subscripts @gol
-Wcomment @gol
-Wduplicate-decl-specifier @r{(C and Objective-C only)} @gol
-Wenum-compare @r{(in C/ObjC; this is on by default in C++)} @gol
-Wformat @gol
+-Wint-in-bool-context @gol
-Wimplicit @r{(C and Objective-C only)} @gol
-Wimplicit-int @r{(C and Objective-C only)} @gol
-Wimplicit-function-declaration @r{(C and Objective-C only)} @gol
@gccoptlist{-Wclobbered @gol
-Wempty-body @gol
-Wignored-qualifiers @gol
+-Wimplicit-fallthrough @gol
-Wmissing-field-initializers @gol
-Wmissing-parameter-type @r{(C only)} @gol
-Wold-style-declaration @r{(C only)} @gol
@code{register}.
@item
-(C++ only) A base class is not initialized in a derived class's copy
-constructor.
+(C++ only) A base class is not initialized in the copy constructor
+of a derived class.
@end itemize
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
+@item -Wformat-length
+@itemx -Wformat-length=@var{level}
+@opindex Wformat-length
+@opindex Wno-format-length
+Warn about calls to formatted input/output functions such as @code{sprintf}
+that might overflow the destination buffer, or about bounded functions such
+as @code{snprintf} that might result in output truncation. When the exact
+number of bytes written by a format directive cannot be determined at
+compile-time it is estimated based on heuristics that depend on the
+@var{level} argument and on optimization. While enabling optimization
+will in most cases improve the accuracy of the warning, it may also
+result in false positives.
+
+@table @gcctabopt
+@item -Wformat-length
+@item -Wformat-length=1
+@opindex Wformat-length
+@opindex Wno-format-length
+Level @var{1} of @option{-Wformat-length} enabled by @option{-Wformat}
+employs a conservative approach that warns only about calls that most
+likely overflow the buffer or result in output truncation. At this
+level, numeric arguments to format directives with unknown values are
+assumed to have the value of one, and strings of unknown length to be
+empty. Numeric arguments that are known to be bounded to a subrange
+of their type, or string arguments whose output is bounded either by
+their directive's precision or by a finite set of string literals, are
+assumed to take on the value within the range that results in the most
+bytes on output. For example, the call to @code{sprintf} below is
+diagnosed because even with both @var{a} and @var{b} equal to zero,
+the terminating NUL character (@code{'\0'}) appended by the function
+to the destination buffer will be written past its end. Increasing
+the size of the buffer by a single byte is sufficient to avoid the
+warning, though it may not be sufficient to avoid the overflow.
+
+@smallexample
+void f (int a, int b)
+@{
+ char buf [12];
+ sprintf (buf, "a = %i, b = %i\n", a, b);
+@}
+@end smallexample
+
+@item -Wformat-length=2
+Level @var{2} warns also about calls that might overflow the destination
+buffer or result in truncation given an argument of sufficient length
+or magnitude. At level @var{2}, unknown numeric arguments are assumed
+to have the minimum representable value for signed types with a precision
+greater than 1, and the maximum representable value otherwise. Unknown
+string arguments whose length cannot be assumed to be bounded either by
+the directive's precision, or by a finite set of string literals they
+may evaluate to, or the character array they may point to, are assumed
+to be 1 character long.
+
+At level @var{2}, the call in the example above is again diagnosed, but
+this time because with @var{a} equal to a 32-bit @code{INT_MIN} the first
+@code{%i} directive will write some of its digits beyond the end of
+the destination buffer. To make the call safe regardless of the values
+of the two variables, the size of the destination buffer must be increased
+to at least 34 bytes. GCC includes the minimum size of the buffer in
+an informational note following the warning.
+
+An alternative to increasing the size of the destination buffer is to
+constrain the range of formatted values. The maximum length of string
+arguments can be bounded by specifying the precision in the format
+directive. When numeric arguments of format directives can be assumed
+to be bounded by less than the precision of their type, choosing
+an appropriate length modifier to the format specifier will reduce
+the required buffer size. For example, if @var{a} and @var{b} in the
+example above can be assumed to be within the precision of
+the @code{short int} type then using either the @code{%hi} format
+directive or casting the argument to @code{short} reduces the maximum
+required size of the buffer to 24 bytes.
+
+@smallexample
+void f (int a, int b)
+@{
+ char buf [23];
+ sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
+@}
+@end smallexample
+@end table
+
@item -Wno-format-zero-length
@opindex Wno-format-zero-length
@opindex Wformat-zero-length
Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
This warning is enabled by @option{-Wall}.
+@item -Wimplicit-fallthrough
+@opindex Wimplicit-fallthrough
+@opindex Wno-implicit-fallthrough
+Warn when a switch case falls through. For example:
+
+@smallexample
+@group
+switch (cond)
+ @{
+ case 1:
+ a = 1;
+ break;
+ case 2:
+ a = 2;
+ case 3:
+ a = 3;
+ break;
+ @}
+@end group
+@end smallexample
+
+This warning does not warn when the last statement of a case cannot
+fall through, e.g. when there is a return statement or a call to function
+declared with the noreturn attribute. @option{-Wimplicit-fallthrough}
+also takes into account control flow statements, such as ifs, and only
+warns when appropriate. E.g.@:
+
+@smallexample
+@group
+switch (cond)
+ @{
+ case 1:
+ if (i > 3) @{
+ bar (5);
+ break;
+ @} else if (i < 1) @{
+ bar (0);
+ @} else
+ return;
+ default:
+ @dots{}
+ @}
+@end group
+@end smallexample
+
+Since there are occasions where a switch case fall through is desirable,
+GCC provides an attribute, @code{__attribute__ ((fallthrough))}, that is
+to be used along with a null statement to suppress this warning that
+would normally occur:
+
+@smallexample
+@group
+switch (cond)
+ @{
+ case 1:
+ bar (0);
+ __attribute__ ((fallthrough));
+ default:
+ @dots{}
+ @}
+@end group
+@end smallexample
+
+C++17 provides a standard way to suppress the @option{-Wimplicit-fallthrough}
+warning using @code{[[fallthrough]];} instead of the GNU attribute. In C++11
+or C++14 users can use @code{[[gnu::fallthrough]];}, which is a GNU extension.
+Instead of the these attributes, it is also possible to add a "falls through"
+comment to silence the warning. GCC accepts a wide range of such comments,
+for example all of "Falls through.", "fallthru", "FALLS-THROUGH" work. This
+comment needs to consist of two words merely, optionally followed by periods
+or whitespaces.
+
+@smallexample
+@group
+switch (cond)
+ @{
+ case 1:
+ bar (0);
+ /* FALLTHRU */
+ default:
+ @dots{}
+ @}
+@end group
+@end smallexample
+
+This warning is enabled by @option{-Wextra}.
+
@item -Wignored-qualifiers @r{(C and C++ only)}
@opindex Wignored-qualifiers
@opindex Wno-ignored-qualifiers
result, but in general it has been found fairly effective at detecting
this sort of problem in programs.
+The C++17 standard will define the order of evaluation of operands in
+more cases: in particular it requires that the right-hand side of an
+assignment be evaluated before the left-hand side, so the above
+examples are no longer undefined. But this warning will still warn
+about them, to help people avoid writing code that is undefined in C
+and earlier revisions of C++.
+
The standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle cases.
Links to discussions of the problem, including proposed formal
@end smallexample
This warning is enabled by @option{-Wall}.
+@item -Wbool-operation
+@opindex Wno-bool-operation
+@opindex Wbool-operation
+Warn about suspicious operations on expressions of a boolean type. For
+instance, bitwise negation of a boolean is very likely a bug in the program.
+For C, this warning also warns about incrementing or decrementing a boolean,
+which rarely makes sense. (In C++, decrementing a boolean is always invalid.
+Incrementing a boolean is invalid in C++1z, and deprecated otherwise.)
+
+This warning is enabled by @option{-Wall}.
+
@item -Wduplicated-cond
@opindex Wno-duplicated-cond
@opindex Wduplicated-cond
width specifiers @code{I32}, @code{I64}, and @code{I} used on Windows targets,
which depend on the MS runtime.
+@item -Waligned-new
+@opindex Waligned-new
+@opindex Wno-aligned-new
+Warn about a new-expression of a type that requires greater alignment
+than the @code{alignof(std::max_align_t)} but uses an allocation
+function without an explicit alignment parameter. This option is
+enabled by @option{-Wall}.
+
+Normally this only warns about global allocation functions, but
+@option{-Waligned-new=all} also warns about class member allocation
+functions.
+
@item -Wplacement-new
@itemx -Wplacement-new=@var{n}
@opindex Wplacement-new
@opindex Wlogical-not-parentheses
@opindex Wno-logical-not-parentheses
Warn about logical not used on the left hand side operand of a comparison.
-This option does not warn if the RHS operand is of a boolean type. Its
-purpose is to detect suspicious code like the following:
+This option does not warn if the right operand is considered to be a boolean
+expression. Its purpose is to detect suspicious code like the following:
@smallexample
int a;
@dots{}
The restrictions on @code{offsetof} may be relaxed in a future version
of the C++ standard.
+@item -Wint-in-bool-context
+@opindex Wint-in-bool-context
+@opindex Wno-int-in-bool-context
+Warn for suspicious use of integer values where boolean values are expected,
+such as conditional expressions (?:) using non-boolean integer constants in
+boolean context, like @code{if (a <= b ? 2 : 3)}. Or left shifting in
+boolean context, like @code{for (a = 0; 1 << a; a++);}.
+This warning is enabled by @option{-Wall}.
+
@item -Wno-int-to-pointer-cast
@opindex Wno-int-to-pointer-cast
@opindex Wint-to-pointer-cast
-findirect-inlining @gol
-fipa-cp @gol
-fipa-cp-alignment @gol
+-fipa-bit-cp @gol
-fipa-sra @gol
-fipa-icf @gol
-fisolate-erroneous-paths-dereference @gol
-fstrict-aliasing -fstrict-overflow @gol
-ftree-builtin-call-dce @gol
-ftree-switch-conversion -ftree-tail-merge @gol
+-fcode-hoisting @gol
-ftree-pre @gol
-ftree-vrp @gol
-fipa-ra}
and loop exit test optimizations.
This option is enabled by default.
-@item -funsafe-loop-optimizations
-@opindex funsafe-loop-optimizations
-This option tells the loop optimizer to assume that loop indices do not
-overflow, and that loops with nontrivial exit condition are not
-infinite. This enables a wider range of loop optimizations even if
-the loop optimizer itself cannot prove that these assumptions are valid.
-If you use @option{-Wunsafe-loop-optimizations}, the compiler warns you
-if it finds this kind of loop.
-
@item -funconstrained-commons
@opindex funconstrained-commons
This option tells the compiler that variables declared in common blocks
them around calls. This is only possible if called functions are part of
same compilation unit as current function and they are compiled before it.
-Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
+Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}, however the option
+is disabled if generated code will be instrumented for profiling
+(@option{-p}, or @option{-pg}) or if callee's register usage cannot be known
+exactly (this happens on targets that do not expose prologues
+and epilogues in RTL).
@item -fconserve-stack
@opindex fconserve-stack
Perform reassociation on trees. This flag is enabled by default
at @option{-O} and higher.
+@item -fcode-hoisting
+@opindex fcode-hoisting
+Perform code hoisting. Code hoisting tries to move the
+evaluation of expressions executed on all paths to the function exit
+as early as possible. This is especially useful as a code size
+optimization, but it often helps for code speed as well.
+This flag is enabled by default at @option{-O2} and higher.
+
@item -ftree-pre
@opindex ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag is
This flag is enabled by default at @option{-O2} and @option{-Os}. It
requires that @option{-fipa-cp} is enabled.
+@item -fipa-bit-cp
+@opindex -fipa-bit-cp
+When enabled, perform ipa bitwise constant propagation. This flag is
+enabled by default at @option{-O2}. It requires that @option{-fipa-cp}
+is enabled.
+
@item -fipa-icf
@opindex fipa-icf
Perform Identical Code Folding for functions and read-only variables.
Disabled at level @option{-Os}.
+@item -fprintf-return-value
+@opindex fprintf-return-value
+Substitute constants for known return value of formatted output functions
+such as @code{sprintf}, @code{snprintf}, @code{vsprintf}, and @code{vsnprintf}
+(but not @code{printf} of @code{fprintf}). This transformation allows GCC
+to optimize or even eliminate branches based on the known return value of
+these functions called with arguments that are either constant, or whose
+values are known to be in a range that makes determining the exact return
+value possible. For example, both the branch and the body of the @code{if}
+statement (but not the call to @code{snprint}) can be optimized away when
+@code{i} is a 32-bit or smaller integer because the return value is guaranteed
+to be at most 8.
+
+@smallexample
+char buf[9];
+if (snprintf (buf, "%08x", i) >= sizeof buf)
+ @dots{}
+@end smallexample
+
+The @option{-fprintf-return-value} option relies on other optimizations
+and yields best results with @option{-O2}. It works in tandem with the
+@option{-Wformat-length} option. The @option{-fprintf-return-value}
+option is disabled by default.
+
@item -fno-peephole
@itemx -fno-peephole2
@opindex fno-peephole
also use other heuristics to decide whether if-conversion is likely to be
profitable.
+@item max-rtl-if-conversion-predictable-cost
+@item max-rtl-if-conversion-unpredictable-cost
+RTL if-conversion will try to remove conditional branches around a block
+and replace them with conditionally executed instructions. These parameters
+give the maximum permissible cost for the sequence that would be generated
+by if-conversion depending on whether the branch is statically determined
+to be predictable or not. The units for this parameter are the same as
+those for the GCC internal seq_cost metric. The compiler will try to
+provide a reasonable default for this parameter using the BRANCH_COST
+target macro.
+
@item max-crossjump-edges
The maximum number of incoming edges to consider for cross-jumping.
The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
optimizers.
When profile feedback is available (see @option{-fprofile-generate}) the actual
-recursion depth can be guessed from probability that function recurses via a
-given call expression. This parameter limits inlining only to call expressions
-whose probability exceeds the given threshold (in percents).
+recursion depth can be guessed from the probability that function recurses
+via a given call expression. This parameter limits inlining only to call
+expressions whose probability exceeds the given threshold (in percents).
The default value is 10.
@item early-inlining-insns
always try to remove unnecessary ivs from the set
when adding a new one.
+@item avg-loop-niter
+Average number of iterations of a loop.
+
@item scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer.
Large expressions slow the analyzer.
To disable use-after-return detection use
@option{--param asan-use-after-return=0}.
+Note: The check is disabled by default at runtime. To enable the check,
+you should set environment variable @env{ASAN_OPTIONS} to
+@code{detect_stack_use_after_return=1}.
+
@item asan-instrumentation-with-call-threshold
If number of memory accesses in function being instrumented
is greater or equal to this number, use callbacks instead of inline checks.
The maximum number of may-defs we analyze when looking for a must-def
specifying the dynamic type of an object that invokes a virtual call
we may be able to devirtualize speculatively.
+
+@item max-vrp-switch-assertions
+The maximum number of assertions to add along the default edge of a switch
+statement during VRP. The default is 10.
@end table
@end table
To optimize the program based on the collected profile information, use
@option{-fprofile-use}. @xref{Optimize Options}, for more information.
+@item -fprofile-update=@var{method}
+@opindex fprofile-update
+
+Alter the update method for an application instrumented for profile
+feedback based optimization. The @var{method} argument should be one of
+@samp{single} or @samp{atomic}. The first one is useful for single-threaded
+applications, while the second one prevents profile corruption by emitting
+thread-safe code.
+
+@strong{Warning:} When an application does not properly join all threads
+(or creates an detached thread), a profile file can be still corrupted.
+
@item -fsanitize=address
@opindex fsanitize=address
Enable AddressSanitizer, a fast memory error detector.
the available options are shown at startup of the instrumented program. See
@url{https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags}
for a list of supported options.
+The option can't be combined with @option{-fsanitize=thread}.
@item -fsanitize=kernel-address
@opindex fsanitize=kernel-address
environment variable; see
@url{https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags} for a list of
supported options.
+The option can't be combined with @option{-fsanitize=address}
+and/or @option{-fsanitize=leak}.
@item -fsanitize=leak
@opindex fsanitize=leak
Enable LeakSanitizer, a memory leak detector.
-This option only matters for linking of executables and if neither
-@option{-fsanitize=address} nor @option{-fsanitize=thread} is used. In that
-case the executable is linked against a library that overrides @code{malloc}
+This option only matters for linking of executables and
+the executable is linked against a library that overrides @code{malloc}
and other allocator functions. See
@uref{https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer} for more
details. The run-time behavior can be influenced using the
@env{LSAN_OPTIONS} environment variable.
+The option can't be combined with @option{-fsanitize=thread}.
@item -fsanitize=undefined
@opindex fsanitize=undefined
Currently this feature only works for @option{-fsanitize=undefined} (and its suboptions
except for @option{-fsanitize=unreachable} and @option{-fsanitize=return}),
@option{-fsanitize=float-cast-overflow}, @option{-fsanitize=float-divide-by-zero},
+@option{-fsanitize=bounds-strict},
@option{-fsanitize=kernel-address} and @option{-fsanitize=address}.
For these sanitizers error recovery is turned on by default, except @option{-fsanitize=address},
for which this feature is experimental.
Syntax without explicit @var{opts} parameter is deprecated. It is equivalent to
@smallexample
--fsanitize-recover=undefined,float-cast-overflow,float-divide-by-zero
+-fsanitize-recover=undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
@end smallexample
@noindent
Similarly @option{-fno-sanitize-recover} is equivalent to
@smallexample
--fno-sanitize-recover=undefined,float-cast-overflow,float-divide-by-zero
+-fno-sanitize-recover=undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
@end smallexample
@item -fsanitize-undefined-trap-on-error
extra information to be omitted and is useful when comparing two assembler
files.
+The added comments include:
+
+@itemize @bullet
+
+@item
+information on the compiler version and command-line options,
+
+@item
+the source code lines associated with the assembly instructions,
+in the form FILENAME:LINENUMBER:CONTENT OF LINE,
+
+@item
+hints on which high-level expressions correspond to
+the various assembly instruction operands.
+
+@end itemize
+
+For example, given this C source file:
+
+@smallexample
+int test (int n)
+@{
+ int i;
+ int total = 0;
+
+ for (i = 0; i < n; i++)
+ total += i * i;
+
+ return total;
+@}
+@end smallexample
+
+compiling to (x86_64) assembly via @option{-S} and emitting the result
+direct to stdout via @option{-o} @option{-}
+
+@smallexample
+gcc -S test.c -fverbose-asm -Os -o -
+@end smallexample
+
+gives output similar to this:
+
+@smallexample
+ .file "test.c"
+# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
+ [...snip...]
+# options passed:
+ [...snip...]
+
+ .text
+ .globl test
+ .type test, @@function
+test:
+.LFB0:
+ .cfi_startproc
+# test.c:4: int total = 0;
+ xorl %eax, %eax # <retval>
+# test.c:6: for (i = 0; i < n; i++)
+ xorl %edx, %edx # i
+.L2:
+# test.c:6: for (i = 0; i < n; i++)
+ cmpl %edi, %edx # n, i
+ jge .L5 #,
+# test.c:7: total += i * i;
+ movl %edx, %ecx # i, tmp92
+ imull %edx, %ecx # i, tmp92
+# test.c:6: for (i = 0; i < n; i++)
+ incl %edx # i
+# test.c:7: total += i * i;
+ addl %ecx, %eax # tmp92, <retval>
+ jmp .L2 #
+.L5:
+# test.c:10: @}
+ ret
+ .cfi_endproc
+.LFE0:
+ .size test, .-test
+ .ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
+ .section .note.GNU-stack,"",@@progbits
+@end smallexample
+
+The comments are intended for humans rather than machines and hence the
+precise format of the comments is subject to change.
+
@item -frecord-gcc-switches
@opindex frecord-gcc-switches
This switch causes the command line used to invoke the
@item pre
@opindex fdump-tree-pre
-Dump trees after partial redundancy elimination. The file name is made
-by appending @file{.pre} to the source file name.
+Dump trees after partial redundancy elimination and/or code hoisting.
+The file name is made by appending @file{.pre} to the source file name.
@item fre
@opindex fdump-tree-fre
Dump each function after Value Range Propagation (VRP). The file name
is made by appending @file{.vrp} to the source file name.
+@item early vrp
+@opindex fdump-tree-evrp
+Dump each function after Early Value Range Propagation (EVRP). The file name
+is made by appending @file{.evrp} to the source file name.
+
@item oaccdevlow
@opindex fdump-tree-oaccdevlow
Dump each function after applying device-specific OpenACC transformations.
Makes the compiler print some statistics about the time consumed by each
pass when it finishes.
+@item -ftime-report-details
+@opindex ftime-report-details
+Record the time consumed by infrastructure parts separately for each pass.
+
@item -fira-verbose=@var{n}
@opindex fira-verbose
Control the verbosity of the dump file for the integrated register allocator.
@option{-march=armv8-a+crc} enables code generation for the ARMv8-A
architecture together with the optional CRC32 extensions.
+@option{-march=armv8.1-a} enables compiler support for the ARMv8.1-A
+architecture. This also enables the features provided by
+@option{-march=armv8-a+crc}.
+
+@option{-march=armv8.2-a} enables compiler support for the ARMv8.2-A
+architecture. This also enables the features provided by
+@option{-march=armv8.1-a}.
+
+@option{-march=armv8.2-a+fp16} enables compiler support for the
+ARMv8.2-A architecture with the optional FP16 instructions extension.
+This also enables the features provided by @option{-march=armv8.1-a}
+and implies @option{-mfp16-format=ieee}.
+
@option{-march=native} causes the compiler to auto-detect the architecture
of the build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the auto-detect
@item -mpic-data-is-text-relative
@opindex mpic-data-is-text-relative
-Assume that each data segments are relative to text segment at load time.
-Therefore, it permits addressing data using PC-relative operations.
-This option is on by default for targets other than VxWorks RTP.
+Assume that the displacement between the text and data segments is fixed
+at static link time. This permits using PC-relative addressing
+operations to access data known to be in the data segment. For
+non-VxWorks RTP targets, this option is enabled by default. When
+disabled on such targets, it will enable @option{-msingle-pic-base} by
+default.
@item -mpoke-function-name
@opindex mpoke-function-name
an option used only for regression testing of the compiler and not
intended for ordinary use in compiling code. This option is disabled
by default.
+
+@item -mpure-code
+@opindex mpure-code
+Do not allow constant data to be placed in code sections.
+Additionally, when compiling for ELF object format give all text sections the
+ELF processor-specific section attribute @code{SHF_ARM_PURECODE}. This option
+is only available when generating non-pic code for ARMv7-M targets.
+
@end table
@node AVR Options
-mpowerpc-gpopt -mpowerpc-gfxopt -msingle-float -mdouble-float @gol
-msimple-fpu -mstring -mmulhw -mdlmzb -mmfpgpr -mvsx @gol
-mcrypto -mdirect-move -mhtm -mpower8-fusion -mpower8-vector @gol
--mquad-memory -mquad-memory-atomic -mmodulo -mfloat128 -mfloat128-hardware @gol
--mpower9-fusion -mpower9-vector -mpower9-dform}
+-mquad-memory -mquad-memory-atomic -mfloat128 -mfloat128-hardware}
The particular options set for any particular CPU varies between
compiler versions, depending on what setting seems to produce optimal
floating point register set that were added in version 2.07 of the
PowerPC ISA. @option{-mupper-regs-sf} is turned on by default if you
use either of the @option{-mcpu=power8}, @option{-mpower8-vector}, or
-@option{-mpower9} options.
+@option{-mcpu=power9} options.
@item -mupper-regs
@itemx -mno-upper-regs
The VSX instruction set (@option{-mvsx}, @option{-mcpu=power7}, or
@option{-mcpu=power8}) must be enabled to use the @option{-mfloat128}
-option. The @code{-mfloat128} option only works on PowerPC 64-bit
+option. The @option{-mfloat128} option only works on PowerPC 64-bit
Linux systems.
+If you use the ISA 3.0 instruction set (@option{-mcpu=power9}), the
+@option{-mfloat128} option will also enable the generation of ISA 3.0
+IEEE 128-bit floating point instructions. Otherwise, IEEE 128-bit
+floating point will be done with software emulation.
+
@item -mfloat128-hardware
@itemx -mno-float128-hardware
@opindex mfloat128-hardware
Enable/disable using ISA 3.0 hardware instructions to support the
@var{__float128} data type.
-@item -mmodulo
-@itemx -mno-modulo
-@opindex mmodulo
-@opindex mno-module
-Generate code that uses (does not use) the ISA 3.0 integer modulo
-instructions. The @option{-mmodulo} option is enabled by default
-with the @option{-mcpu=power9} option.
-
-@item -mpower9-fusion
-@itemx -mno-power9-fusion
-@opindex mpower9-fusion
-@opindex mno-power9-fusion
-Generate code that keeps (does not keeps) some operations adjacent so
-that the instructions can be fused together on power9 and later
-processors.
+If you use @option{-mfloat128-hardware}, it will enable the option
+@option{-mfloat128} as well.
-@item -mpower9-vector
-@itemx -mno-power9-vector
-@opindex mpower9-vector
-@opindex mno-power9-vector
-Generate code that uses (does not use) the vector and scalar
-instructions that were added in version 3.0 of the PowerPC ISA. Also
-enable the use of built-in functions that allow more direct access to
-the vector instructions.
-
-@item -mpower9-dform
-@itemx -mno-power9-dform
-@opindex mpower9-dform
-@opindex mno-power9-dform
-Enable (disable) scalar d-form (register + offset) memory instructions
-to load/store traditional Altivec registers. If the @var{LRA} register
-allocator is enabled, also enable (disable) vector d-form memory
-instructions.
+If you select ISA 3.0 instructions with @option{-mcpu=power9}, but do
+not use either @option{-mfloat128} or @option{-mfloat128-hardware},
+the IEEE 128-bit floating point support will not be enabled.
@item -mfloat-gprs=@var{yes/single/double/no}
@itemx -mfloat-gprs
Overriding the default ABI requires special system support and is
likely to fail in spectacular ways.
+@item -mgnu-attribute
+@itemx -mno-gnu-attribute
+@opindex mgnu-attribute
+@opindex mno-gnu-attribute
+Emit .gnu_attribute assembly directives to set tag/value pairs in a
+.gnu.attributes section that specify ABI variations in function
+parameters or return values.
+
@item -mprototype
@itemx -mno-prototype
@opindex mprototype
@item -mdual-nops
@itemx -mdual-nops=@var{n}
@opindex mdual-nops
-By default, GCC inserts nops to increase dual issue when it expects
+By default, GCC inserts NOPs to increase dual issue when it expects
it to increase performance. @var{n} can be a value from 0 to 10. A
-smaller @var{n} inserts fewer nops. 10 is the default, 0 is the
+smaller @var{n} inserts fewer NOPs. 10 is the default, 0 is the
same as @option{-mno-dual-nops}. Disabled with @option{-Os}.
@item -mhint-max-nops=@var{n}
@opindex mhint-max-nops
-Maximum number of nops to insert for a branch hint. A branch hint must
+Maximum number of NOPs to insert for a branch hint. A branch hint must
be at least 8 instructions away from the branch it is affecting. GCC
-inserts up to @var{n} nops to enforce this, otherwise it does not
+inserts up to @var{n} NOPs to enforce this, otherwise it does not
generate the branch hint.
@item -mhint-max-distance=@var{n}
@itemx -mno-nop-mcount
@opindex mnop-mcount
If profiling is active (@option{-pg}), generate the calls to
-the profiling functions as nops. This is useful when they
+the profiling functions as NOPs. This is useful when they
should be patched in later dynamically. This is likely only
useful together with @option{-mrecord-mcount}.
git://gcc.gnu.org / gcc.git / blobdiff